Mechanisms of 7,8-dihydroneopterin protection of macrophages from cytotoxicity

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Date

Permanent Link

Thesis Discipline

Biochemistry

Degree Grantor

University of Canterbury

Degree Level

Doctoral

Degree Name

Doctor of Philosophy

Gamma-interferon stimulates human macrophages to produce of 7,8-dihydroneopterin (7,8-NP). 7,8-NP and its oxidation product neopterin are excellent inflammatory markers for a variety of chronic conditions, including atherosclerosis. The biological significance of 7,8-NP in atherosclerosis is not fully understood, but 7,8-NP has been shown to protect macrophage cells from oxidised low density lipoprotein (oxLDL). Cellular accumulation of oxLDL-derived lipids and oxLDL-induced cytotoxicity are major drivers of atherosclerotic plaque progression. This thesis investigated the mechanisms of 7,8-NP-mediated protection against oxLDL-induced damage to macrophage cells. The research assessed the relative contribution of the previously identifyed antioxidant capacity of 7,8-NP and its ability to down-regulate oxLDL uptake. OxLDL cytotoxicity was characterised by high intracellular oxidative stress within the first 12 hours of exposure, which was critical to oxLDL toxicity. Exogenously added 7,8-NP effectively scavenged the intracellular oxidants generated in response to oxLDL, shown by the oxidation of 7,8-NP to neopterin. The ability of 7,8-NP to alleviate oxidative stress during the critical time-frame of acute toxicity was the primary mechanism of protection. 7,8-NP was also found to down-regulate the levels of intracellular oxysterol esters in oxLDL-treated macrophages. This decrease was associated with the reduction of CD36 scavenger receptor protein and mRNA expression. The late onset of these processes in the second half of the 24 hour incubation highlighted their potential role in foam cell formation. Research indicated that 7,8-NP may play a role in the reverse cholesterol transport in these cholesterol ester-loaded cells. The CD36 down-regulation by 7,8-NP did not protect macrophages from acute oxLDL cytotoxicity. This research reveals novel detail about the mechanism of 7,8-NP protection of macrophages from cytotoxic effects of oxLDL. It is suggested that 7,8-NP may protect macrophage cells in the atherosclerotic plaques by scavenging ROS produced during acute cytotoxicity and alleviate oxysterol ester accumulation, thus stabilising macrophage cells during chronic oxLDL exposure.